1. Field of the Invention
This invention relates to improvements in impact-type printing machines and more in particular to a printing hammer rebound control for insuring a smooth returning motion of the hammer irrespective of the condition of its forward or impacting motion.
2. Description of the Prior Art
Shown in FIG. 1 is a plunger type printing hammer assembly which is widely used. The assembly comprises a coil 1 supported by a yoke 2, which also supports a pair of bearings 3 and 4. A printing hammer 5 is rotatably and slidably supported by the bearings 3 and 4. The hammer 5 comprises sliding shaft sections 9 and a core section 10, which is generally located inside the coil 1, and an impact surface 5' is formed at its forward end. As shown, the core section 10 is formed somewhat larger in diameter than the sliding shaft sections 9 to provide a shoulder portion, and a coil spring 6 is provided to extend between one end surface of the bearing 3 and the shoulder portion of the core section 10.
When the coil 1 is energized by passing a current therethrough, the hammer 5 is forced to move in the direction indicated by the arrow A and, therefore, the impact surface 5' comes into contact with a type element 7 located at a printing section to cause the type element 7 impact a recording medium (not shown) on a platen 8 thereby forming a printed character on the recording medium. Then, the coil 1 is deenergized with an appropriate timing so that the hammer 5 now moves in the direction indicated by the arrow B because it receives the repulsive force from the platen 8 at the time of impact as well as the recovery force of the spring 6.
However, when the backward shoulder of the core section 10 hits the bearing 4 at the extreme end of the returning motion of the hammer 5, the hammer 5 bounces back and force or vibrates. To cope with such a problem, it has been proposed to provide a damper 11 as shown in FIG. 1. It has, however, been found that such an approach is unsatisfactory for various reasons. For example, use may be made of a rubber damper with ease; however, the damping characteristics deteriorate as the rubber ages.
In the case where the impact energy of the returning hammer 5 is not sufficiently absorbed by the damper 11, the hammer 5 rebounds and moves again in the direction indicated by the arrow A. It may happen that the rebounding hammer 5 again strikes the type element 7, and, alternatively, the forward end of the hammer 5 may be trapped between spokes of the printing wheel which is in rotation to bring the next selected type element to the printing section. In the worst case, the printing wheel may be damaged, necessitating replacement with a new one.
It is true that a hammer rebound may be decreased by decelerating the returning velocity of the hammer 5 when it approaches its original position. Such deceleration may be carried out by reenergizing the coil 1. Such a rebound control concept is illustrated in FIGS. 2(a) and (b). As shown, when a main energization of the coil 1 is carried out by an electric current having waveform A shown in FIG. 2(b), the hammer 5 displaces as shown by the solid line in FIG. 2(a). At the tail end of the displacement curve, the amount of rebound is indicated by S. However, if the coil 1 is reenergized during the returning motion of the hammer 5 by an electric current having waveform B shown in FIG. 2(b), the hammer 5 is decelerated at its final step thereby decreasing the amount of rebound as shown by the dotted line in FIG. 2(a).
It should, however, be noted that different type characters require different impact forces in order to attain uniform print density along an entire line of different characters. Moreover, it is sometimes required to print some characters or words with a different print density from the other characters or words in order to highlight the informational context of the document being printed. One such typical prior art printing system which can provide a variable print hammer striking force capable of being adjusted over a wide range of magnitudes is disclosed in the U.S. Pat. No. 4,118,129, issued to Grundherr on Oct. 3, 1978. Under such conditions, the above-described rebound control approach required further refinement and improvements.